A historical method of providing an etched translucent image on a window or transparent objet d'art is to protect all the surrounding surfaces with an intimately bonded, temporary masking material having the negative image of the intended image. The exposed image area is then eroded away via abrasive projectile impingement or chemical erosion, to generate an irregular surface. Finally, the masking material is removed. The translucence created by diffusion of the light impinging on the irregular surface contrasts with light which is transmitted by or specularly reflected from the adjacent transparent area resulting in a decorative image.
The common viewing condition for etched transparent articles such as a window glass is via its second surface, (i.e., when the observer is standing outside, looking through the first surface of the window at the image etched on the inside or second surface). One practical reason for having the image on the inside is that if the etched surface would become wet with rain water the image would become distorted because the diffusing surface would be covered by water. For this outside viewing condition the image that is etched on the inside of the window is the reverse or mirror image of the desired viewing image. Another, more aesthetic reason for commonly viewing the etched image through the glass is the uniform and high surface gloss of the first surface. It is desirable that a decorating method meant to approximate the aesthetic characteristics of etched glass is compatible with the common viewing condition. Etching of window glass, high refractive index glass, or transparent polymeric plastic via the above methods results in an image that has the same milky-white, translucent color in all viewing conditions, regardless of illumination and observation angle, (i.e., no display of prismatic color is evident).
The above-described historical etching method is disadvantageous in that it is time consuming, labor intensive, and results in an etched appearance which is not reversible. For this reason other methods have been developed which imitate the appearance of etching, but which are more economical and potentially reversible. One such method is to bond a clear adhesive plastic film having a screen printed ink image thereon which simulates the appearance of an etched surface by having the ink contain opacifying pigments and/or flattening pigments, such as is taught in British Patent No. 2,254,042. Another method found commercially involves the use of a pressure-sensitive adhesive coated plastic film containing fillers and flattening agents which approximate an etched appearance. This film is provided on a release liner and can be cut via computer driven X-Y cutters, separated from its background image, and applied as a transfer graphic to a glass or plastic surface to provide an etched appearance. Removal could be accomplished via solvent soak or scraping with a sharp edge. As with historical etching methods, neither method provides a prismatic color display under any condition of illumination or observation.
For creating decorative prismatic color displays there are a variety of methods. The historical method involves the use of a prism, which is a high refractive index glass having a surface consisting of angularly cut facets. Typically, the prism is placed where it can be illuminated by sunlight, such as by hanging in front of a window where sunlight is present. The light passing through the prism is dispersed into its visible wavelengths and projected onto a viewing surface such as an interior wall. Because of the multiple facets, multiple images are projected. These images have rainbow-like color progressions. Rainbow-like images are also visible within the prism itself, reflected off the interior faceted surfaces. Faceted prisms can be affixed on one or more surfaces to enhance their decorative appeal, but as relatively large, discrete objects, prisms are not practical for decorating continuous surfaces. Rather, they are typically employed as spot decorations.
Diffraction gratings are another method of creating a decorative prismatic display. White light falling on a reflecting surface having surface imperfections which are about the size of a wavelength of light of the visible spectrum will be spread around and past the obstacles, providing a colorful display which can be used for decorative effects. Typically, the diffraction grating is embossed into the second surface of a transparent polymeric film and then covered with a thin layer of a reflecting mirror such as via silver reduction or vapor deposited aluminum. Holographic imaging techniques can provide more complex decorative images having a three-dimensional appearance. Most commercial decorative holographic displays are opaque because of the reflecting surface; however, JP 53-07347 describes such a device that uses a transparent reflecting surface instead of metal.
The primary disadvantages of such diffraction based decorative devices is that the requirement for an internal or second reflecting surface makes it very difficult to simultaneously create a translucent decorative etched appearance which approximates the traditional methods.